Method and arrangement for measuring the vapor pressure of liquids

ABSTRACT

Method for measuring vapor pressure of liquids in which liquid to be measured is placed in a previously-evacuated measuring cell, and influence of the gas dissolved in the liquid on the measuring results is eliminated. The liquid to be measured is added in at least two separate portions, with total pressure present in the measuring cell being measured after the addition of each portion and after saturation vapor pressure equilibrium has become established, to obtain a thus-measured value. Gas pressure present in the measuring cell which is due to the gas dissolved in the liquid is derived from at least two thus-measured values which had been obtained at the same measuring temperature in which pressure components due to vapor pressure of the liquid are equal, taking into account the liquid quantity contained in the particular liquid portions and the measuring cell volume with the relation po=RT. The vapor pressure of the liquid is determined by subtracting the derived gas pressure from the total measured pressure. The present invention is also directed to an arrangement for measuring the vapor pressure of liquids, which has a measuring cell and a pressure measuring device connected to the same, as well as to a filling tube and a vacuum connection. The pressure measuring device is connected with the measuring cell through a connecting tube extending nearly to a bottom of the measuring cell. The opening of the connecting tube thereby lies below a liquid level present during the measurement in the measuring cell.

BACKGROUND OF THE INVENTION

The invention relates to a method for measuring the vapor pressure ofliquids in which method the liquid to be measured is placed into apreviously evacuated measuring cell and the effect of the gasesdissolved in the liquid on the measurement results is excluded. Theinvention, further, relates to an arrangement for measuring the vaporpressure of liquids.

The vapor pressure of a liquid is that pressure which in a closed systembuilds up above the liquid; this pressure changes until the vapor is indynamic equilibrium with the liquid. The then-present pressure is calledsaturation vapor pressure and this is exclusively a function of thetemperature of the system and does not depend on its geometry. Thevalues published in the literature for the vapor pressure of a substanceare always saturation vapor pressure values, since only these can bestated specifically for the substance and do not depend on the measuringmethod. In principle, the measurement of the vapor pressure of a liquidcan be carried out so that this liquid is placed into a tightly closablevessel which is provided with a pressure measuring arrangement, thevessel closed, and the pressure present in the vessel is measured. Ifthe quantity of liquid is sufficiently large so that at the measuringtemperature in spite of evaporation residual liquid remains in thevessel, the vapor pressure is the saturation vapor pressure. Thesecircumstances, which in principle are simple, are complicated by thefact that the ambient atmospheric pressure and the pressure of the gasdissolved in the liquid to be measured can significantly influence themeasurement.

The techniques used until now either accept a falsification of themeasured results through the ambient atmospheric pressure and thepressure of the gas dissolved in the liquid, with in some cases the aimbeing to keep the error approximately the same through standardizedprocedural specifications in order to be able to compare the measuringresults obtained from different liquids (for example vapor pressuredetermination after Reid), or the aim is with extremely great operationcomplexity in terms of laboratory means to exclude not only theinfluence of the ambient atmospheric pressure but also to free theliquid to be tested of the gases dissolved therein before undertakingmeasurements. With the known techniques particularly great difficultiesare encountered if the vapor pressure of mixtures of liquids is to bedetermined.

In GB-A-832 085 a method for measuring the vapor pressure of liquids isdescribed in which first the liquid to be tested is placed into anevacuated chamber in which grainy material is located in order to makeavailable a large surface. The large surface favors the emergence ofgases dissolved in the liquid. Following this pretreatment, the liquidis transferred to a bottle, the evacuated chamber is cleaned, and theliquid is subsequently placed once again into said chamber whichthereupon is again evacuated and subsequently closed, whereupon thepressure developing in this chamber is measured. This approach iscumbersome, and in the case of liquid mixtures changes in thecomposition result through the evacuation. It is in GB-A-832 085 furtherdescribed that the quantity of a gas contained in a liquid can bedetermined if the vapor pressure of this liquid can be neglected, inthat this liquid is placed into an evacuated chamber in which grainymaterial is present, and measures the pressure built up in this chamberby the gas emerging from the liquid. From the thereby, obtained measuredvalues for the pressure using the relation p*v=R*T, the quantity of thegas which had been present in the liquid relative to normal pressure andnormal temperature is calculated.

A method for determining the gas content of a liquid is also describedin GB-A-974 183. In this method too, the liquid to be tested is placedinto an evacuated chamber, and the pressure originating in this chamberis measured. The volume of the chamber in which the liquid is located isvariable by displacing a piston. Two measurements at different positionsof the piston are carried out. The measured pressure is composed in eachinstance from the pressure of the gas and the vapor pressure of theliquid. From two measured pressure values, assuming that the unknownvapor pressure has the same value in both measurements, the gas quantitycontained in the liquid is calculated.

SUMMARY OF THE INVENTION

It is a goal of the present invention to create a method for measuringthe vapor pressure of liquids which is simple to carry out and which iswell suited for industrial application, and in so doing supplies precisemeasuring results. The invention is further to create an arrangementwhich permits simple execution of the method according to the inventionand permits achieving good measuring accuracy.

The method according to the invention of the initially cited type ischaracterized in that the liquid to be measured is placed, in two orseveral portions, into the previously evacuated measuring cell, with thesecond liquid portion following the first liquid portion or thefollowing liquid portions being in each instance added to the liquidalready present in the measuring cell. In each instance after theplacing of a portion after the saturation vapor pressure equilibrium hasbecome established, the total pressure present in the measuring cell ismeasured. From at least two of the measured values obtained in theprocess at identical measuring temperature, in which the pressurecomponents originating from the vapor pressure of the liquid are equaltaking into account the liquid quantities contained in the particularliquid portions and the measuring cell volume using the relationp*v=R*T, the vapor pressure present in the measuring cell is derivedwhich stems from the gas which had been dissolved in the liquid to betested. The vapor pressure of the liquid is determined by subtractingthis gas pressure from the measured total pressure. In this way thevalue of the absolute saturation vapor pressure is obtained.

Through the approach according to the invention the above stated goalcan readily be addressed and it is a significant advantage of thismethod that it also permits precise measurement of the saturation vaporpressure of mixtures of liquids. The method according to the inventionis particularly advantageous for measuring the vapor pressure ofhydrocarbons which may contain relatively large quantities of dissolvedgases, including air.

In the method according to the invention through the addition inportions of the liquid to be measured into the measuring cell and bycarrying out at least two measurements at the same temperature, however,at different filling levels of the measuring cell at least two measuringvalues are formed in which the pressure component formed by the vaporpressure of the liquid in each instance is equally large. Using thisspecific property of the measured values as basis the pressure componentstemming from the gases is derived, and after knowing the latterpressure components the vapor pressure of the liquid can be determinedfrom the total pressure obtained at each measurement.

It is advantageous in the interest of a simple process execution if theliquid to be measured is added to the measuring cell in portions ofequal size with respect to each other.

To increase the accuracy of the measuring results it is favorable if fordetermining the gas pressure, several pressure measurements are carriedout, each time at the same temperature.

A very simple derivation of the gas pressure within the scope of themethod according to the invention results if the liquid to be measuredis added to the measuring cell in two portions of equal size, if the twopressure measurements following the addition of the portions into themeasuring cell are undertaken at the same temperature if the gaspressure occurring after the addition of the second liquid portion isdetermined with the relation p_(G) =2(p₂ -p₁), where p₁ and p₂ are themeasured values obtained in the first and the second pressuremeasurement. To increase the accuracy, a correction factor is to be usedin determining the gas pressure which takes into account the compressionwhich originates through the introduction of the additional portion ofthe liquid to be measured.

A further embodiment of the method according to the invention, whichpermits in a simple manner, a precise determination of the curve ofvapor pressure of liquids over a temperature range, is characterized inthat after the measurement undertaken to determine the gas pressure,additional pressure measurements are carried out at differenttemperatures and for the determination of the vapor pressure of theliquid to be measured obtained at these temperatures, a value of the gaspressure is subtracted from the obtained measured values corrected tothe temperature at the particular measurement using the relationp*v=R*T.

For carrying out the method according to the invention, a simpleclosable measuring cell can be used which is equipped with a pressuremeasuring arrangement and which can be brought to predeterminedtemperatures with a thermostat. After cleaning and evacuating thismeasuring cell to a final pressure of below 1 hPa by means of anapportioning syringe which can be placed on a filling tube neck of themeasuring cell, the first half of the liquid to be measured can be addedto the measuring cell. In the measuring cell thereupon a total pressureresults which is composed of the vapor pressure of the liquid and thegas pressure of the gases which had been dissolved in the added liquidportion. After the temperature adjustment this total pressure ismeasured. Subsequently the second half of the liquid is placed into themeasuring cell with the apportioning syringe and again temperatureadaptation and development of the saturation vapor pressure equilibriumwaited for, whereupon again the total pressure obtained in the measuringcell is measured. The second measurement is carried out at the sametemperature as the first measurement. Because the temperature is thesame, the saturation vapor pressure of the liquid or the component ofthe total pressure formed by the saturation vapor pressure is the samein both measurements. In the second measurement the gas pressurecorresponds to the total quantity of gases which were dissolved in thetwo liquid portions which were placed into the measuring cell.

Thus, in filling two equal portions of the liquid to be measured intothe measuring cell and at the same temperature the followingcorrelations result during the first and the second pressuremeasurement:

p₁ =p_(F1) +p_(G) (m): total pressure in the measuring cell at liquidquantity m

p₂ =p_(F1) +p_(G) (2m): total pressure in the measuring cell at liquidquantity 2m

where

p_(F1) : is the vapor pressure of the liquid

p_(G) (m): the gas pressure of the gases which had been dissolved in theliquid quantity m

p_(G) (2m): the gas pressure of the gases which had been dissolved inthe liquid quantity 2m.

As a first approximation on the basis of the gas equation p*v=R*T atconstant temperature

    p.sub.G (2m)=2(p.sub.G (m))

can be set with sufficient accuracy.

If a more precise derivation of the gas pressure is desired, the gascompression occuring upon adding the second liquid portion is taken intoaccount in a correction factor which results from the known volume ofthe measuring cell and the volume of the added liquid portions.

From the above relation a simple derivation of the gas pressure resultsas follows:

    p.sub.2 -p.sub.1 =(p.sub.F1 +p.sub.G (2m))-(p.sub.F1 +p.sub.G (m))

in which p₁ can, as above, be corrected using a correction factor. With:

    p.sub.G (2m)=2p.sub.G (m)

    p.sub.2 -p.sub.1 =p.sub.G (2m)-p.sub.G (m)

results and

    p.sub.G (2m)=2(p.sub.2 -p.sub.1)

is obtained.

The gas pressure p_(G) (2m) derived in this manner which stems from thetotal liquid quantity present in the measuring cell is subtracted fromthe measured total pressure in order to obtain the vapor pressure p_(F1)of the liquid.

In analogous manner the liquid to be measured can be placed in more thantwo portions into the measuring cell and a pressure measurement can becarried out at the same temperature after each portion is added and fromthese measurements, in a manner analogous to the previously described inconjunction with two pressure measurements, the gas pressure can bederived. The derivation of this gas pressure from more than twomeasurements is more complex than the derivation from two total pressuremeasurements described previously. It does, however, permit greateraccuracy.

As soon as the gas pressure has been derived, the temperature present inthe measuring cell can be changed in order to determine the vaporpressure at any given temperatures. The previously obtained value of thegas pressure for these other measuring temperatures is corrected orconverted using the gas equation p*v=R*T and subtracted from themeasured total pressure values, in order to obtain the saturation vaporpressure of the tested liquid at the particular measuring temperatures.In this way a complete vapor pressure curve which also detects a greatertemperature range can easily be determined.

The arrangement according to the invention for measuring the vaporpressure of liquids, which has a measuring cell and a pressure measuringdevice connected to this measuring cell as well as a filling tube and avacuum connection, is characterized in that the pressure measuringdevice is connected to the measuring cell through a connecting tubereaching nearly to the bottom of the measuring cell, and so the openingof the connecting tube during the measurement lies below the liquidlevel provided in the measuring cell. This design of the arrangementpermits that with very simple structure a good accuracy of the pressuremeasurements required for the vapor pressure determination can beachieved. The pressure measuring device can be kept separate from themeasuring cell at essentially constant operating temperature (inparticular at ambient temperature), whereby in a very simple mannerimpairments of accuracy of the pressure measurement which may originatedue to different temperatures at the pressure measuring device areexcluded. By virtue of that fact that the connecting tube which leadsfrom the measuring cell to the pressure measuring device opens under theliquid level in the measuring cell, penetration of vapors of the liquidinto the connecting tube and into the pressure measuring device isexcluded. Therewith falsification of the measured values which mayoriginate through a condensation of these vapors in the connecting lineor in the pressure measuring device is prevented.

A preferred embodiment of the arrangement implemented according to theinvention which permits a very simple operation of this arrangement ischaracterized in that to a connecting tube leading from the measuringcell to the pressure measuring device, a selector valve is connectedfrom which extend the filling tube and the vacuum connection.

A very simple structure and also a simple operatability of thearrangement results, if the measuring cell is arranged in a solid phasethermostat. Thereby good heat contact of thermostat to measuring celland also easy accessibility of the measuring cell during operation isobtained if the measuring cell is conically shaped along its outside inthe longitudinal direction and the solid phase thermostat has a metalblock which has a bore shaped corresponding to the outside of themeasuring cell into which can be disposed the measuring cell.

Further, with very simple constructional setup good settability of thetemperature over a very wide temperature range results which reachesfrom values lying relatively far below the ambient temperature to hightemperatures, if provisions are made that the solid phase thermostat hasPeltier elements for setting the temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in greater detail with reference toan embodiment example of an arrangement implemented according to theinvention and represented in the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the arrangement represented in the drawing, a measuring cell 1 isprovided which advantageously consists of stainless and acid resistantspecial steel. Such a measuring cell may, for example, have a volume of5 ml and be formed in the shape of a cylinder with an inner diameter of10 mm. The measuring cell 1 is closed with a sealing cover 2 which canbe screwed on, with a sealing ring being advisably inserted between thesealing cover 2 and the measuring cell 1 to provide sealing. Through thesealing cover 2 a temperature sensor 3 is inserted into the interior ofthe measuring cell 1; such a temperature sensor is advisably arranged ina special steel hollow needle which projects into the liquid in themeasuring cell so that the temperature sensor detects precisely thetemperature of this liquid. Through the sealing cover 2, further, aconnecting tube 4 which is also advantageously implemented in the formof a hollow needle is inserted into the interior of the measuring cell 1and projects into the bottom area of the measuring cell. The connectingtube 4 leads to a measuring head 5 in which a pressure measuring device6 is disposed. The pressure measuring device can advantageously beimplemented in the form of a piezoresistive pressure transducer which isinstalled in such a way that the free volume is of minimum size. On theupper side of the measuring head 5 a selector valve 7 is closely placedextending the connecting tube 4, which selector valve is implemented inthe form of a three-way ball valve. From the selector valve 7 extend avacuum connection 8 and a filling tube neck 9. Thus via the selectorvalve 7 and the vacuum connection 8 the measuring cell 1 can beevacuated, and via the filling tube neck 9 be filled with the liquid tobe tested. For filling, for example, an apportioning syringe can beplaced in the filling tube neck 9. Thereby that the connecting tube 4reaches to the bottom of the measuring cell 1, and hence opens below theliquid level present in the measuring cell 1 during operation,penetration of gases from the gas space of the measuring cell into theconnecting tube 4 and further to the pressure measuring device 6 can becounteracted and such errors of measurement are avoided which, forexample, may result due to the occurrence of condensation phenomena inthe area of the connecting tube 4 and in the area of the pressuremeasuring device 6. The measuring cell 1 is shaped conically at itsoutside 14 in the longitudinal direction and seats in a correspondinglyshaped bore 15 which is provided in a metal block 10, preferably analuminum block, of a solid phase thermostat. In the metal block 10 forcontrolling or setting the temperature of the same, a temperature sensor13 is arranged which controls Peltier elements 11 via a control devicenot further shown. The Peltier elements 11 are thermically coupled withcooling bodies 12 and permit setting the temperature of the metal block10 and hence also setting the temperature which obtains in the measuringcell 1 to the particular desired value. With the aid of these Peltierelements the temperature of the metal block 10 can be set in any givenway to a value within the range of -20° C. to 150° C.

Advantageously an automatic control of the operating processes can beprovided with the aid of a control apparatus which sequentially controlsthe measurements carried out in the course of the filling process takingplace by portions and also automatically the derivation of thesaturation pressure value from the measurements of the determined totalpressure values. In measuring vapor pressure curves with automaticcontrol of the progression of the filling and measuring processesthrough the thereby given, fixed function progression, the temperaturechanges can be carried out either in discrete steps or also continuouslywith settable rate. The measured values can be automatically sensed,stored, and printed out and, as mentioned, from the measured valuesderive automatically the vapor pressure values and also store and printout the vapor pressure values, if desired. It is also possible to useapparatus which automatically plots curves (plotter) to representautomatically vapor pressure curves.

I claim:
 1. Method for measuring vapor pressure of liquid in whichliquid to be measured is placed in a previously-evacuated measuring celland influence of gas dissolved in the liquid on measuring results iseliminated, comprising the steps ofadding the liquid to be measured intwo or more separate portions into the previously-evacuated measuringcell, measuring total pressure present in the measuring cell followingeach addition of one portion and after saturation vapor pressureequilibrium has become established, to obtain a thus-measured value,deriving gas pressure present in the measuring cell due to the gasdissolved in the liquid from at least two thus-measured values obtainedat the same measuring temperature, in which pressure components due tovapor pressure of the liquid are equal, taking into account liquidquantity contained in the liquid portions and the measuring cell volumeand using the relationship pv=RT, and determining vapor pressure of theliquid by subtracting the thus-derived gas pressure from the totalmeasured pressure.
 2. The method of claim 1, wherein the liquid to bemeasured is added to the measuring cell in portions which aresubstantially equal.
 3. The method of claim 1, comprising the additionalstep ofcarrying out several pressure measurements at the sametemperature, to determine the gas pressures.
 4. The method of claim 1,whereinthe liquid to be measured is placed in the measuring cell in twoportions of substantially equal size, and the gas pressure occurringafter the addition of the second subsequent liquid portion is determinedby the relationship p_(G) =2(p₂ -p₁), wherein p₁ and p₂ are the measuredvalues obtained as the first and second measured values.
 5. The methodof claim 4, comprising the additional step of,utilizing a correctionfactor in the determination of the gas pressure, for taking into accountcompression which occurs in the placing of the additional portion ofliquid to be measured in the cell.
 6. The method of claim 1, comprisingthe additional steps ofcarrying out further pressure measurements atdifferent temperatures, to obtain further measured values, andsubtracting from the further measured values that were obtained, a valueof gas pressure corrected to the temperature at the particularmeasurement using the relationship pv=RT, for determining the vaporpressure of the liquid at these different tempeatures.
 7. Apparatus formeasuring vapor pressure of liquids, comprisinga measuring cell (1), apressure measuring device (6) coupled to said measuring cell (1) and toa filling tube (9) and a vacuum connection (8), wherein said pressuremeasuring device (6) is coupled to said measuring cell (1) through aconnecting tube (4) extending near to a bottom of the measuring cell(1), with an opening of said connecting tube (4) arranged to lie belowliquid level present in said measuring cell (1) during measurement. 8.The combination of claim 7, additionally comprisinga selector valve (7)coupled to said connecting tube (4) extending from said measuring cell(1) to said pressure measuring device (6), such that said filling tube(9) and said vacuum connection (8) both extend from said selector valve(7).
 9. The combination of claim 7, additionally comprising a solidphase thermostat in which said measuring cell (1) is arranged.
 10. Thecombination of claim 9, wherein said measuring cell (1) is conicallyshaped along an outer surface (14) thereof in a longitudinal direction,andsaid solid phase thermostat comprises a metal block (10) having abore (15) complementary-shaped to the outer surface of said measuringcell (1), such that said measuring cell (1) can be disposed saidthermostat bore (15).
 11. The combination of claim 9, wherein said solidphase thermostat comprises Peltier elements (11) for setting thetemperature.